In a micro electro mechanical system (MEMS), the development of a sacrificial layer technique has become a key factor for manufacturing a suspended structure, such as a cantilever, a beam, a membrane, a channel, a cavity, a joint or hinge, a link, a crank, a gear or a rack, to name a few. A structure release etching process is adapted for removing a sacrificial layer, so a structure of a structure release in a micro electro mechanical system has a critical influence on the process of removing the sacrificial layer.
A conventional structure release etching process is first introduced with an interference display cell as an example. The interference display cell, a kind of a micro electro mechanical system, is used to fabricate a planar display. Planar displays have great superiority in the portable display device and limited-space display market because they are lightweight and small. To date, in addition to liquid crystal displays (LCD), organic electro-luminescent displays (OLED), and plasma display panels (PDP), a mode of optical interference displays is another option for planar displays.
U.S. Pat. No. 5,835,255 discloses an array of display cells of visible light that can be used in a planar display. Referring to FIG. 1, FIG. 1 illustrates a cross-sectional view of a conventional display cell.
Every optical interference display cell 100 comprises two walls, wall 102 and wall 104. The wall 102 and the wall 104 are supported by supporters 106, and a cavity 108 is subsequently formed between the wall 102, the wall 104 and the supporters 106. The distance between the wall 102 and the wall 104, that is, the length of the cavity 108, is D. Either the wall 102 or the wall 104 is a semi-transmissible/semi-reflective layer with an absorption rate that partially absorbs visible light, and the other is a light reflective layer that is deformable when voltage is applied. When the incident light passes through the wall 102 or the wall 104 and into the cavity 108, in wavelengths (λ) of all visible light spectra of the incident light, only visible light with a wavelength λ1 corresponding to formula 1.1 can generate a constructive interference and can be emitted, that is,2D=Nλ  (1.1)where N is a natural number.
When the length D of the cavity 108 is equal to half of the wavelength multiplied by any natural number, a constructive interference is generated and a sharp light wave is emitted. In the meantime, if an observer follows the direction of the incident light, a reflected light with wavelength λ1 can be observed. Therefore, the optical interference display cell 100 is “open”.
FIG. 2 illustrates a cross-sectional view of a conventional display cell after a voltage is applied. Referring to FIG. 2, while driven by the voltage, the wall 104 is deformed and falls down towards the wall 102 due to the attraction of static electricity. At this time, the distance between the wall 102 and the wall 104, that is, the length of the cavity 108, is not exactly equal to zero, but is d, which can be equal to zero. If D in formula 1.1 is replaced with d, only visible light with a wavelength λ2 satisfying formula 1.1 in wavelengths λ of all visible light spectra of the incident light can generate a constructive interference, be reflected by the wall 104, and pass through the wall 102. Because the wall 102 has a high light absorption rate for light with wavelength λ2, all the incident light in the visible light spectrum is filtered out and an observer who follows the direction of the incident light cannot observe any reflected light in the visible light spectrum. Therefore, the optical interference display cell 100 is now “closed”.
FIG. 3A and FIG. 3B illustrate a method for manufacturing a conventional display cell. Referring to FIG. 3A, a first electrode 110 and a sacrificial layer 111 are formed in sequence on a transparent substrate 109, and opening 112, which is suitable for forming a supporter therein, is formed in the first electrode 110 and the sacrificial layer 111. Then, a supporter 106 is formed in the opening 112. Next, an electrode 114 is formed on the sacrificial layer 111 and the supporter 106. Subsequently, referring to FIG. 3B, the sacrificial layer 111 shown in FIG. 3A is removed by a release etching process to form a cavity 116, which is located in the position of the sacrificial layer 111, and the length D of the cavity 116 is the thickness of the sacrificial layer 111.
In a micro electro mechanical process, a micro suspended structure is fabricated by use a sacrificial layer. A suspended movable microstructure is fabricated by a selective etching between a device structure layer and the sacrificial layer to remove the sacrificial layer and leave the structure layer, and this process is called a structure release etching. The difference between the structure release etching process and an IC process is that in the structure release etching process, the selective etching is an isotropic etching, so that an undercut or an under etching is formed in the structure layer for smooth separation of the structure layer and the substrate.
The most popular structure release etching process is a wet structure release process. In the wet structure release process, a rinsing step and a drying step usually have to be performed after etching, and a microstructure can substantially be suspended above the substrate. However, during the wet structure release process, it is quite easy for the structure and the substrate to stick together, thereby resulting in failure of the device. A dry etching process using xenon difluoride (XeF2) as an etchant can be used to solve the problems resulted in the wet etching process.
Xenon difluoride is in a solid state at normal temperature and normal pressure, and is sublimated into the gaseous state at low pressure. Xenon difluoride has great etching selectivity on silicon materials, such as monocrystalline silicon, polysilicon and amorphous silicon, and some metals, such as molybdenum (Mo), molybdenum alloy and so on. Xenon is an inert gases, and xenon difluoride is quite unstable. The etching mechanism of xenon difluoride is that two fluorine free radicals are brought to the reaction positions by xenon, and when xenon difluoride contacts the material to be etched, xenon difluoride decomposes to release these two fluorine free radicals. Because the isotropic etching effect of xenon difluoride is great, xenon difluoride has an excellent capacity for lateral etching. In a micro electro mechanical system process, xenon difluoride is used as an etchant to remove a sacrificial layer in a structure release etching process.
Referring to FIG. 4, FIG. 4 illustrates a top view of a conventional optical interference display cell. The optical interference display cell 200 includes separation structures 202, such as defined by dotted lines 2021, located on two opposite sides of the optical interference display cell 200, and supporters 204 located on the other two opposite sides of the optical interference display cell 200. The separation structures 202 and the supporters 204 are located between two electrodes. There are gaps between the supporters 204, and the supporters 204 and the separation structures 202. The gaseous xenon difluoride permeates through the gaps and etches a sacrificial layer (not shown in FIG. 4). The rate of a structure release etching process with an etchant of the gaseous xenon difluoride changes with the different materials of the sacrificial layers desired to be etched. Typically, the etching rate can be greater than 10 micrometers per minute, and even can be up to 20-30 micrometers per minute for some materials. For the size of the present optical interference display cell, one structure release etching process only takes dozens of seconds to 3 minutes.
Although the structure release etching process performed with the etchant of gaseous xenon difluoride has the aforementioned advantages, a disadvantage of the high cost of the structure release etching process results from the character of xenon difluoride itself. Xenon difluoride is expensive, and is particularly sensitive to moisture and is unstable. When xenon difluoride contacts moisture, hydrogen fluoride is produced. Hydrogen fluoride is not only dangerous, but also reduces efficiency of etching. Besides, the structure release etching process performed using xenon difluoride as an etchant is rare in semiconductor processes and typical planar display processes, so etchers that are maturely developed in the semiconductor processes and the liquid crystal display processes are unsuitable for the structure release etching process with xenon difluoride etchant. The process apparatuses used in semiconductor or typical planar display can be continuously used in most of the main processes of the optical interference display, but the structure release etching process needs a totally different apparatus design. To reorganize and consolidate the process apparatuses would be an obstacle to the development and throughput of the optical interference display.